This invention relates in general to electrophotography and, in particular, to an electrophotographic imaging member.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an optional overcoating layer.
Imaging members are generally exposed to repetitive electrophotographic cycling which subjects the exposed charge transport layer thereof to abrasion, chemical attack, heat and multiple exposures to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed charge transport layer. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
In a production web stock of several thousand feet of coated multilayered photoreceptor roll-up, the charge transport layer and the anti-curl layer are in intimate contact. The high surface contact friction of the charge transport layer against the anti-curl layer causes dimples and creases to develop in the internal layers of the photoreceptor. These physically induced defects are seen to manifest themselves into print defects in xerographic copies.
When ultrasonically welded into a belt, seams in some multilayered photoresponsive imaging members can delaminate during fabrication when larger webs are slit into smaller belt size sheets. Further, after the sheets are welded into belts, the belts tend to delaminate during extended cycling over small diameter support rollers or when subjected to lateral forces caused by rubbing contact with stationary web edge guides during cycling Seam delamination is further aggravated when the belt is employed in electrophotographic imaging systems utilizing blade cleaning devices. In addition, belt delamination is encountered during web slitting operations to fabricate belt photoreceptors from wide webs. Alteration of materials in the various belt layers such as the conductive layer, hole blocking layer, adhesive layer, charge generating layer, and/or charge transport layer to reduce delamination is not easily effected because the new materials may adversely affect the overall electrical, mechanical and other properties of the belt such as residual voltage, background, dark decay, flexibility and the like.
U.S. Pat. No. 4,869,982 discloses an electrophotographic photoreceptor containing a toner release material in a charge transport layer. From about 0.5 to about 20 percent of a toner release agent selected from stearates, silicon oxides and fluorocarbons is incorporated into a charge transport layer.
U.S. Pat. No. 4,784,928 to Kan et al discloses an electrophotographic element having two charge transport layers. An outermost charge transport layer or overcoating may comprise a waxy spreadable solid, stearates, polyolefin waxes, and fluorocarbon polymers such as Vydax fluorotelomer from du Pont and Polymist F5A from Allied Chemical Company.
U.S. Pat. No. 4,664,995 discloses an electrostatographic imaging member utilizing a ground strip. The disclosed ground strip material comprises a film forming binder, conductive particles and microcrystalline silica particles dispersed in the film forming binder, and a reaction product of a bi-functional chemical coupling agent which interacts with both the film forming binder and the microcrystalline silica particles.
U.S. Pat. No. 4,717,637 discloses a microcrystalline silicon barrier layer.
U.S. Pat. Nos. 4,678,731 and 4,713,308 disclose microcrystalline silicon in the photoconductive and barrier layers of a photosensitive member.
U.S. Pat. No. 4,675,262 discloses a charge transport layer containing powders having a different refractive index than that of the charge transport layer excluding the powder material The powder materials include various metal oxides.
U.S Pat. No. 4,647,521 discloses the addition of amorphous hydrophobic silica powder to the top layer of a photosensitive member. The silica is of spherical shape and has a size distribution between 10 and 1000 Angstroms. Hydrophobic silica is a synthetic silica having surface silanol (SiOH) groups replaced by hydrophobic organic groups such as --CH.sub.3.
If a relatively great frictional force acts between the photosensitive member and a cleaning member, the surface of the photosensitive member may be damaged, and wear-off or filming of the toner may result due to the high surface contact friction between the cleaning device and the charge transport layer of the photosensitive member Wear in the photosensitive member surface caused by high frictional force during machine function reduces the thickness of the charge transport layer. This reduction in charge transport layer thickness increases the electrical field across the layer, and alters electrophotographic performance. Moreover, static electricity generated by friction results in nonuniform surface potential in the charging step, which in turn causes an irregular image formation or fogging. In order to reduce the frictional force, the pressure of the cleaning member, e.g., a cleaning blade, may be reduced. However, by reducing the frictional force, the cleaning blade may not be able to clean the photosensitive member sufficiently, resulting in toner build-up or surface filming.
Other attempts at reducing the frictional force acting between the cleaning blade and the photosensitive member include adding a lubricant such as wax to the toner. However, the fixability of the toner may degrade its electrical function, or further filming may occur, resulting in a degraded image.
A further proposal for reducing frictional force involves applying a lubricant on the surface of the photosensitive drum. U.S. Pat. No. 4,519,698 discloses a waxy lubricant method to constantly lubricate a cleaning blade. However, the thickness of the lubricant film formed on the photosensitive drum cannot be maintained, and interference with the electrostatic characteristics of the photosensitive member occurs. Attempts have also been made to construct a cleaning blade with a material having a low coefficient of friction. However, these attempts are subject to the problem of degradation in other characteristics, especially mechanical strength, due to the presence of additives.
Another problem in multilayered belt imaging systems includes cracking in one or more critical imaging layers during belt cycling over small diameter rollers. Cracks developed in the charge transport layer during cycling are a frequent phenomenon and are most problematic because they can manifest themselves as print-out defects which adversely affect copy quality. Charge transport layer cracking has a serious impact on the versatility of a photoreceptor and reduces its practical value.
When one or more photoconductive layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. An anti-curl layer may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the tendency to curl.
It is desirable to increase the durability and extend the life of the exposed charge transport layer surface in an imaging device as well as to reduce frictional contact between members of the imaging device while maintaining electrical and mechanical integrity.